1. Field
This invention generally relates to container handling gantry cranes including ship to shore, rubber tire gantry, and rail mounted gantry.
2. Prior Art
Every year the ships and cranes get bigger and faster, but the means for loading stay the same. The driver of a tractor pulling a chassis with a container on it pulls under a crane, two or more workers on the ground uncouple the container from the chassis, and the load is lifted onto the ship. The container is coupled to the chassis by strong elliptical toggles in each corner which are rotated in elliptical sockets in the container. Sometimes the chassis is not fully uncoupled and the container, chassis, tractor, and operator are lifted into the air. This usually results in the tractor breaking free and falling to the ground causing injuries to the operator and damage to the equipment. Because the tractors have no rear springs, a drop from as little as 200 mm can jostle the operator and cause neck and back injuries, for this reason an improved system is needed.
Several attempts have been made to resolve this problem. U.S. Pat. No. 5,260,688 issued Nov. 9, 1993 used a user selectable radio transmitter to deactivate the crane if the tractor was rotated around the front wheel and a wand contacted the ground. This system had several drawbacks. The wand could be easily damaged by hitting debris in the container yard. Also by the time the wand contacted the ground the rear of the tractor could be up to 800 mm off of the ground. While it would protect the tractor from being lifted entirely off of the ground, the driver could still be injured. A careless operator could improperly select the radio channel, or forget to altogether. This could leave the tractor unprotected. It could also shut down an adjacent crane while it is in motion causing the crane operator to lose control of the load endangering workers on the ship and dock.
U.S. Pat. No. 5,455,567 describes a system using a photo sensor on the rear of the tractor to trigger a 28 Hz strobe light. The strobe is picked up by a pulse discriminator mounted on the cranes trolley that inhibits the crane hoist. The system is active any time the tractor is running. This system worked fairly well when it was developed but is not adequate for the newer, faster cranes unless slowdowns are added. Photo sensors are unreliable on asphalt. Paint stripes and other color changes can change their set points and result in a delayed hoist deactivation. The reaction time between the strobe and the pulse discriminator can be as high as 500 ms. Modern cranes have hoist speeds of 190 m per minute and accelerate to full speed in 2.0 seconds. If you take into account the strobe does not signal until the tractors rear wheels have lifted to about 150 mm and the crane continues to accelerate, the tractors rear wheels can be 800 to 1500 mm off of the ground before the hoist is stopped. For this reason the later versions of this system added a 2 second slowdown to the crane that limited the hoist speed to 20% of the base speed when hoisting a container from the dock. The later systems also abandoned the photo sensor and used a single axis tilt sensor to trigger the strobe. The single axis tilt sensor still did not detect the lift until the tractor had been lifted about 200 mm off of the ground. Even with the added slow down a perfectly operating system did not stop the hoist until the tractors rear wheels were about 300 mm off of the ground. The single axis tilt sensor was also ineffective at detecting roll that can happen if only one corner is coupled.